1. Field of the Invention
This invention relates generally to paving materials and more particularly to an apparatus and method for mixing asphalt and rubber to form an asphalt-rubber pavement surfacing and repairing composition.
2. Description of the Prior Art
The increased volume of traffic along with general aging has caused severe problems on many roadways, streets, and other paved surfaces in this country. A particular problem results from elastic type failures in pavements which causes cracking patterns of the type sometimes referred to as an "alligator" cracking pattern. This type of elastic failure is caused by fatigue of the pavement surface resulting from repeated deflections. Other problems of concern are random cracking of paved surfaces due primarily from expansion and contraction, and the general aging of the sealing materials in expansion joints. All of these types of pavement failures must be repaired to prevent water and/or uncompressible materials from entering into the cracks or joints. If water enters into such cracks or joints it can either wash out the base materials, or cause a general breaking up of the pavement due to freezing. If uncompressible materials, such as sand, enter into the cracks or joints it will severely restrict expansion of the pavement and again result in a general breaking up of the pavement surface.
Considerable experimentation has been conducted in recent times to provide relatively low cost repair techniques, and of particular interest is an asphalt-rubber composition which has proven to be a very satisfactory material for use as a seal coating, for filling and sealing random cracks, as a replacement for deteriorated materials in expansion joints, and the like.
Briefly, the asphalt-rubber composition is a reaction product which is neither asphalt nor rubber in nature but is an elastomeric sealing compound. The asphalt-rubber composition includes a mixture of paving grade asphalt and granulated crumb rubber of the non-oil resistant asphalt-soluble type, which are prepared and mixed in conformity to a specific method and mixture proportions. The asphalt is heated to a range of between 350.degree. F. to 500.degree. F. and the granulated rubber is added thereto, and mixed together. Although the mix proportions may vary somewhat, it has been found that mix proportions of between 2 to 3 parts of asphalt and one part of rubber are satisfactory and that a mix proportion of 75% plus or minus 2% of asphalt by weight and 25% plus or minus 2% of rubber provides the ideal composition which possesses an ideal balance between the sealing characteristics of the asphalt and the elasticity of the rubber. This particular asphalt-rubber composition is fully disclosed in U.S. Pat. No. 3,891,585 issued on June 24, 1975 to Charles H. McDonald.
Although the asphalt-rubber composition is an excellent material, its more widespread usage has been held back by problems with handling and mixing of the asphalt and rubber materials.
The asphalt material is normally delivered in bulk form, such as in tank cars, to the mixing site and other than some occassional and relatively minor contamination, will not pose any problems in addition to the well known and expected difficulties associated with the handling of such material.
The rubber generally used for this purpose is obtained from a supplier who grinds up old automobile tires and packages the granulated crumb rubber in bags of predetermined weight for shipment to the mixing site. The rubber supplier is responsible for removing all metal and other contaminants from the rubber, and for the most part does a pretty good job. However, some metal, primarily from ground up steel belted radial tires, will be found in the granulated rubber, and when this occurs, the useful life of pumps and other equipment used in the handling and application of the asphalt-rubber composition will be severely shortened.
Although the above described contamination of the asphalt and rubber materials can be detrimental to the finished product and the handling equipment, the inherent characteristics of those materials pose the biggest problem, in that it is very difficult to mix those materials and produce a consistently blended mixture of the proper proportions. The granulated rubber is a somewhat cohesive material and as such will often form clods that block supply conduits, applicator devices and the like. In addition, the rubber has a tendency to float and avoid mixing with the asphalt.
In general, the prior art practice for mixing the asphalt and rubber materials includes pumping the molten asphalt (350.degree. F.-500.degree. F.) through a suitable flow meter into a mixing tank and manually adding an appropriate number of bags of granulated rubber. The flow meter is used to control the amount of asphalt that is pumped into the mixing tank, and the amount of rubber is controlled by counting the number of pre-weighed bags of rubber that are added to the tank. Although this method of determining the mix proportions is rather crude, if carefully done, the results can be quite satisfactory. However, the packaging, handling, weighing, and particularly shipping of the individual bags of granulated rubber is very costly and time consuming, but is indispensable as far as the prior art technique of mixing is concerned, in that the individual bags of predetermined weight are relied upon for controlling the mix proportions.
The prior art mixing tanks, although varying somewhat in configuration, are all basically the same. Briefly, the mixing tanks are elongated horizontally disposed structures with some sort of an agitation device such as an auger arrangement which extends longitudinally through the bottom of the tank. The asphalt and rubber materials are introduced into the mixing tank through suitable ports and conduits located at the top thereof, and the mixed asphalt-rubber composition exits the tank by means of a pump and conduit arrangement located at the bottom of the tank. A typical prior art mixing structure of the above described type is fully shown and described in U.S. Pat. No. 3,610,588 issued on Oct. 5, 1971 to G. W. Diefenbach.
The above described prior art practice and mixing mechanisms have proven less than satisfactory for several reasons. First, the prior art makes no provisions for ridding the asphalt and granulated rubber of contaminants. Secondly, the manual introduction of the granulated rubber into the mixing tank is of course, subject to human error and improper formulation of the asphalt-rubber composition can occur. Thirdly, the prior art mixing apparatus will not always break up the lumps or clods of granulated rubber and this can cause plugging of the conduits and applicator devices. The fourth, and most troublesome problem with the prior art method and the apparatus, is that the produced asphalt-rubber composition is not always a consistently blended mixture of the proper mix proportions.
As previously mentioned, the granulated rubber has a tendency to float and avoid mixing with the molten asphalt, and therefore, the upper portion of the materials within the tank will have a somewhat larger concentration of rubber than the materials in the lower part of the tanks. During draining the mixture having a lower rubber concentration will be pumped out faster and easier than that having a high concentration of rubber. Since the asphalt-rubber composition is pumped out of the bottom of the mixing tank, the floating rubber will coat the interior of the tank, and the residual composition remaining in the tank after draining, will have a high rubber concentration. It is very rare for a mixing tank to be used for mixing a single batch of the asphalt-rubber composition in that production and/or job requirements most often require very large quantities of the composition.
The above described coating of the tank and residual concentrations will have a cumulative effect and it has been estimated that rubber concentrations will reach between 30% and 35% near the end of a day's continuous mixing tank usage, and this, of course, can cause serious problems with the integrity of the asphalt-rubber composition.
Therefore, a need exists for a new and improved method and apparatus for mixing an asphalt-rubber composition which overcomes some of the problems and shortcomings of the prior art.